diff --git a/Cargo.toml b/Cargo.toml
index ad6c2a6..c52b2f4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,13 +17,18 @@ simple-error = "0.3.1"
 wasm-bindgen = "0.2.95"
 wasm-logger = "0.2.0"
 web-sys = { version = "0.3.72", features = [
+    "CssRule",
+    "CssRuleList",
     "CssStyleDeclaration",
+    "CssStyleSheet",
     "Document",
     "Element",
     "HtmlCollection",
     "HtmlDivElement",
     "Location",
     "MouseEvent",
+    "StyleSheet",
+    "StyleSheetList",
     "Touch",
     "TouchEvent",
     "TouchList",
diff --git a/src/ui/buttons.rs b/src/ui/buttons.rs
index c712aa4..396f155 100644
--- a/src/ui/buttons.rs
+++ b/src/ui/buttons.rs
@@ -1,5 +1,5 @@
 use wasm_bindgen::prelude::*;
-use web_sys::{window, HtmlElement, MouseEvent};
+use web_sys::{window, CssStyleSheet, HtmlElement, MouseEvent};
 
 use crate::{
     board::{get_empty_field_idx, get_shuffle_sequence},
@@ -145,6 +145,7 @@ fn get_optimal_solve_callback(size: usize) -> Closure<dyn FnMut(MouseEvent)> {
             return;
         }
 
+        // TODO: Solver aborting after a certain size?
         let ids = BOARD.with_borrow(|b| b.board().indices2ids().clone());
         match find_swap_order(&ids, size, size) {
             Ok(solve_sequence) => {
@@ -228,3 +229,50 @@ fn apply_solve_sequence(solve_sequence: Vec<(usize, usize)>, interval: i32) {
         .unwrap();
     finish_callback.forget();
 }
+
+const ACTIVE_BUTTON_COLOR: &str = "rgb(233, 233, 237)";
+const INACTIVE_BUTTON_COLOR: &str = "rgb(208, 208, 215)";
+
+pub(crate) fn deactivate_buttons() {
+    replace_in_button_style(&[
+        ("pointer", "wait"),
+        (ACTIVE_BUTTON_COLOR, INACTIVE_BUTTON_COLOR),
+    ]);
+}
+
+pub(crate) fn activate_buttons() {
+    replace_in_button_style(&[
+        ("wait", "pointer"),
+        (INACTIVE_BUTTON_COLOR, ACTIVE_BUTTON_COLOR),
+    ]);
+}
+
+fn replace_in_button_style(replaces: &[(&str, &str)]) {
+    let document = window().unwrap().document().unwrap();
+    let style_sheets = document.style_sheets();
+
+    // The global style sheet is the first one.
+    if let Some(global_sheet) = style_sheets.get(0) {
+        let global_css = global_sheet.dyn_into::<CssStyleSheet>().unwrap();
+        let rule_list = global_css.css_rules().unwrap();
+
+        // Iterate rules until we find the rule for the button class.
+        for i in 0..rule_list.length() {
+            match rule_list.get(i) {
+                Some(rule) if rule.css_text().contains("button {") => {
+                    let existing_rule = rule.css_text();
+                    let updated_rule = replaces
+                        .iter()
+                        .fold(existing_rule, |rule, (old, new)| rule.replace(old, new));
+                    log::debug!("Updating to {updated_rule}");
+                    // Replace existing with updated rule in two steps.
+                    // `replace` would be better but returns a `Promise` which we do not want to handle in this context.
+                    global_css.delete_rule(i).unwrap();
+                    global_css.insert_rule(&updated_rule).unwrap();
+                    return;
+                }
+                _ => (),
+            }
+        }
+    }
+}
diff --git a/src/ui/mod.rs b/src/ui/mod.rs
index ccfc0c0..140e3c3 100644
--- a/src/ui/mod.rs
+++ b/src/ui/mod.rs
@@ -1,3 +1,5 @@
+use buttons::{activate_buttons, deactivate_buttons};
+
 use crate::UI_LOCKED;
 
 pub(crate) mod board;
@@ -23,6 +25,7 @@ pub(crate) fn lock_ui() -> bool {
             false
         } else {
             *locked = true;
+            deactivate_buttons();
             log::debug!("Locked UI");
             true
         }
@@ -35,6 +38,7 @@ pub(crate) fn unlock_ui() {
             log::warn!("Should unlock UI which was not locked");
         } else {
             *locked = false;
+            activate_buttons();
             log::debug!("Unlocked UI");
         }
     })
@@ -43,6 +47,3 @@ pub(crate) fn unlock_ui() {
 pub(crate) fn ui_locked() -> bool {
     UI_LOCKED.with(|locked| *locked.borrow())
 }
-
-// TODO: Change button colors when locking UI.
-// TODO: Solver not attempting / button greyed out at a certain size
diff --git a/www/index.html b/www/index.html
index bde8c4b..0a0b056 100644
--- a/www/index.html
+++ b/www/index.html
@@ -2,7 +2,7 @@
 <html lang="en">
   <head>
     <meta content="text/html;charset=utf-8" http-equiv="Content-Type" />
-    <link rel="stylesheet" href="./style.css" />
+    <link href="./style.css" rel="stylesheet" type="text/css" />
   </head>
   <body>
     <div class="content">
diff --git a/www/style.css b/www/style.css
index f2c2c40..6a1965f 100644
--- a/www/style.css
+++ b/www/style.css
@@ -11,10 +11,11 @@ button {
   border-radius: 20px;
   padding: 0.5rem;
   cursor: pointer;
+  background-color: rgb(233, 233, 237);
 }
 
-.clickable:hover {
-  cursor: pointer;
+button:hover {
+  background-color: rgb(208, 208, 215);
 }
 
 .content {